1. Field of the Invention
The present invention relates to an enhanced tunnel field effect transistor, and in particular to an enhanced tunnel field effect transistor with an epitaxial tunnel layer therein.
2. Description of the Prior Art
In recent years, the green energy issue is attracted a lot of attention for many people. As the MOSFET (Metal Oxide Semiconductor Field Effect Transistor) scales down rapidly, a large number of devices result in high standby and dynamic power consumption. To eliminate this undesirable phenomenon, lowering the off-state leakage current and operating voltage are the two intuitive methods to achieve these demands. As the reduction of the leakage current and operating voltage, steep S.S. is required to achieve sufficient on-state current to drive the circuits. In conventional MOSFET, the operation principle of thermionic carrier injection limits from the steep S.S. to 60 mV/decade at the room temperature. This physical constraint limits the scaling of threshold voltage and the operating voltage.
The tunnel FET (tunnel field effect transistor) becomes an appropriate candidate of such low power application device due to the tunnel FET with intrinsic operation characteristics. The distinct operation mechanism, band-to-band tunneling (BTBT), is applied to a tunnel FET device which breaks the limitation of 60 mV/decade and makes the opportunities to push the low power application into a new generation. However, the steep S.S. in tunnel FET only occurs at very low current levels and degrades rapidly due to inefficient BTBT. To improve the tunneling efficiency, low band-gap material introduction has been proposed to lower the tunnel barrier.
Some groups applied low band-gap material only at the source side to enhance the tunneling current. Although it is a great idea to enhance the on-state current and to keep low off-state current, the device is very hard to be implemented and the defect issues at the hetero junction interface is also a serious problem. In addition, the tunnel barrier is usually located in the intrinsic region. It is more intuitive to replace the intrinsic region material instead of the source material.
Other groups used the low band-gap material to replace the whole channel region. This methodology needs to consider the bipolar BTBT effect which could result in higher off-state current. Novel structure of the tunnel FET also has been proposed continuously to improve the performance of tunnel FET. All of them need to control a special doping profile such as ultrathin doping region and ultrahigh doping concentration. It is hard to be realized and may results in large variations in fabrication.